78 ESSENTIALS OF BOTANY 



The medullary rays in the young shoot serve as a chan- 

 nel for the transference of water and plant-food in a liquid 

 form across the stem, and they often contain much stored 

 food. 



The vessels carry water upward and (sometimes) air 

 downward through the stem. 



The wood-cells of the heartwood are useful only to give 

 stiffness to the stem. Those of the sapwood, in addition 

 to this work, have to carry most of the water from the 

 roots to the leaves and other distant portions of the plant. 



The cambium layer is the region in which the annual 

 growth of the tree takes place. 



The most important portion of the inner bark is that 

 which consists of sieve-tubes, for in these digested and 

 elaborated plant-food is carried from the leaves toward 

 the roots. 



The green layer of the bark in young shoots does much 

 toward collecting nutrient substances, or raw materials, 

 and preparing the food of the plant from air and water, 

 but this work may be best explained in connection with 

 the study of the leaf (Chapter XIII). 



93. Movement of Water in the Stem. — The student has 

 already learned that large quantities of water are taken 

 up by the roots of plants. 



Having become somewhat acquainted with the structure 

 of the stem, he is now in a position to investigate the 

 question as to how the various fluids, commonly known as 

 sap, travel about in it.^ It is important to notice that sap 

 is by no means the same substance everywhere and at all 

 times. As it first makes its way by osmotic action inward 



1 See the paper on The So-called Sap of Treen and its Movements, by 

 Professor Charles E. Barnes, Science, Vol. XXI, p. 535. 



